Method of improving parallelism of a die to package using a modified lead frame

ABSTRACT

A structure and method are provided to allow a die to be packaged more uniformly and in parallel with a package by utilizing a lead frame having at least one cavity within the lead frame, thereby allowing excess die-attach epoxy can flow into the cavity or cavities and reducing the amount of contact surface area between the die and lead frame.

BACKGROUND

1. 1. Field of the Invention

2. This invention relates to the manufacture of semiconductor devices,and more specifically to the process of assembling an integrated circuitinto a package.

3. 2. Related Art

4. Laser fuses have been used in the electronics industry to repairmemory elements, configure logic circuits, and customize integratedcircuits such as gate arrays by selectively removing desired fuses inthe device. For example, a non-functional device can be repaired byremoving desired fuses, e.g., ablation by laser, to isolate defectiveportions of the circuitry or to substitute functionally redundantcircuitry for the defective portions. Fuses can also be used to mark thedevice for identification of characteristics in a manner that isreadable visually or electrically, e.g., serialization of the integratedcircuit or how the device has been configured by the laser. Anintegrated circuit can be customized or configured for specific uses byaltering the structure, path or electrical characteristics of the deviceor elements through selective removal of the fuses. It should be notedthat the word “fuse” can refer to an antifuse as well as a fuse.

5. The fuses can be removed at various stages of the integrated circuitmanufacturing process. By removing the fuses after the wafer has beensawed up into individual integrated circuits and assembled intopackages, the lead time to deliver customized or configured integratedcircuits, hereinafter referred to as die, can be reduced. A typicalprocess for assembling a die into a package and configuring the die inthe package includes the following basic steps:

6. 1. deposit die-attach epoxy on a lead frame in the cavity of thepackage, where the cavity can be formed at the time the package iscreated or at a later time;

7. 2. place the die on the epoxy and compress the die and the packagetogether to distribute the epoxy;

8. 3. bake the package and die to cure the epoxy;

9. 4. attach bond wires between bonding pads on the die and lead fingerson the package;

10. 5. configure the die with the use of a laser; and

11. 6. seal off the top of the package, either with a lid or by fillingin the cavity with mold compound.

12. A major factor limiting the successful application of the laser andconfiguration of the die in this manner is the difficulty with aligningand focusing the laser if the die does not sit evenly on the lead frame.A die 10 typically has alignment and focusing marks or targets 1-4 onthe die's upper surface, as shown in FIG. 1. A typical alignment andfocusing sequence for die 10 would begin with a laser scanning targets 1though 4 one at a time and making x-axis, y-axis, and rotationalcorrections. The laser then returns to target 1 and makes another seriesof scans through to target 4 to set the focus level for the entire die10.

13. Alignment and focusing in this manner is adequate as long as the dieis set evenly upon the lead frame so that the laser scanning plane isparallel with the surface of the die. In other words, as long as anynon-orthogonality in the plane of the die, relative to the optics of thelaser, does not cause the lasered or scanned portion of the die to falloutside of the laser spot's focus range, proper alignment and focusingcan be achieved. However, when the die is not properly placed on thelead frame, the laser may be unable to perform an alignment or focusscan or to accurately ablate the fuses for proper circuit customization.

14.FIG. 2 shows a die 10, which has been placed and compressed on somedie-attach epoxy 20. Epoxy 20 has been deposited on a lead frame (notshown) within a cavity of a package 21. Several factors can cause die 10to sit at an angle relative to package 21, including the presence of airbubbles 22 within epoxy 20, an uneven distribution of epoxy 20, or anuneven placement of die 10 on epoxy 20. This will often result in aportion of die 10 falling out of the focus range of the laser, which canlead to improper configuration of the die.

15. Although various leveling mechanisms are common in the industry andwould be effective in re-leveling the die, these types of mechanisms arenot commonly present in commercially-available lasers used for therepair and configuration of semiconductor devices. Therefore, usingthese mechanisms to re-level the die so that reliable configuration ofthe die in a package is possible would require costly and time-consumingmodifications to the laser. Accordingly, it is desirable to have amethod of accurately adhering-a die to a package so that re-leveling thedie is unnecessary, which eliminates the additional expenses required tomodify the laser for re-leveling.

SUMMARY

16. The present invention provides a structure and method for accuratelyadhering a die to a package by modifying the lead frame in the packageto allow excess die-attach epoxy to distribute itself into at least onecavity in the lead frame and to reduce the surface area of the leadframe supporting the die. As a result, the amount of epoxy between thedie and the lead frame is reduced for less variation in the angle of thedie relative to the package, and the die is provided a more rigid andstable setting to minimize the adverse effects of non-uniform epoxydistribution.

17. In one embodiment of the present invention, the lead frame containsan array of cavities formed in the lead frame, with the array slightlylarger than the size of the die to be packaged. When the die is pressedonto the lead frame and die-attach epoxy, the excess epoxy can spreadinto the cavities. The cavities form a grid in the lead frame whichpermit the die to be well-supported, yet minimizes the surface contactarea, which reduces both the chance of bubble formation and the amountof pressure required to seat the die uniformly on the lead frame. As aresult, a more level positioning of the die relative to the lead frameand the package is possible.

18. In another embodiment of the present invention, a cavity ofgenerally circular or oval shape is formed partially or completelythrough the lead frame prior to die-attach epoxy deposition. Onedimension of the cavity would be slightly smaller than the longerdimension of the die to be packaged. The die-attach epoxy is depositedin the cavity, and the die pressed onto the top of the epoxy and thelead frame. The excess epoxy is forced into unfilled areas of thecavity, and if necessary, through the gaps formed between the edge ofdie and cavity. The die is well-supported and in direct contact with thelead frame so that the die is positioned parallel with respect to thelead frame and thus the package. The die-attach epoxy anchors the diefirmly in position without affecting the position of the die relative tothe lead frame because excess epoxy is not forced between the uppersurface of the lead frame and the die.

19. The present invention permits a more consistent placement of the diein a package which results in better yield for applications where thedie is laser-coded in the package.

20. The present invention will be more fully understood in light of thefollowing detailed description taken together with the accompanyingdrawings.

BRIEF DESCRIPTION OF DRAWINGS

21.FIG. 1 is a top view of the upper surface of a die showingconventional alignment and focusing targets;

22.FIG. 2 is a side view of a die mispositioned with respect to apackage;

23.FIGS. 3A and 3B are top and side views, respectively, of a lead frameaccording to one embodiment of the present invention; and

24.FIGS. 4A and 4B are top and side views, respectively, of a lead frameaccording to another embodiment of the present invention.

25. Note that use of the same reference numbers in different figuresindicates the same or like elements.

DETAILED DESCRIPTION

26. According to the present invention, a structure and method areprovided which allow a die to be accurately adhered to a package bycreating at least one cavity in the lead frame contained in the packageso that the amount of epoxy between the upper surface of the lead frameand the die is reduced, which provides a more rigid and stable settingfor the die. Excess die-attach epoxy flows into the cavity or cavitiesto minimize the effects of non-uniform epoxy distribution along thecontact surfaces of the die and epoxy. As a result, the die can beadhered to the package in a manner which allows improved parallelismbetween the package and die.

27.FIGS. 3A and 3B show one embodiment of the present invention, whereFIG. 3A is a top view of a die 30 set on a lead frame 31, where leadframe 31 is located within a package 32 and contains an array ofrectangular cavities 33. FIG. 3B is a side view of FIG. 3A alongsectional line A-A′. Although shown as rectangular cavities 33, othersuitably shaped cavities, such as circular, can also be used with thisembodiment. The array of cavities 33 formed within lead frame 31 ispreferably slightly larger than the size of the die 30 to be placed onthe lead frame 31, although not required. A larger array allows excessdie-attach-epoxy 34 more areas into which to flow. However, a smallerarray can also be used with a smaller amount of die-attach epoxy. Forexample, the epoxy would only be deposited over the array, and not overother areas of the lead frame.

28. The cavities 33 are cut or formed prior to depositing die-attachepoxy 34, such as at the time the lead frame is manufactured. Aftercavities 33 have been formed, die-attach epoxy 34 is deposited on leadframe 31. Die 30 is then placed and pressed onto lead frame 31 by anysuitable placement mechanism. Excess epoxy 34 spreads into cavities 33underneath lead frame 31 to allow a more stable placement of die 30 ascompared with conventional lead frames that only allow excess epoxy tospread toward the outer edges of the die. Lead frame 31 allows theexcess epoxy to spread across less surface area of the die before thedie is set, which reduces the likelihood of die mispositioning due tothe effects of epoxy spreading.

29. Forming the lead frame 31 with cavities 33 also forms a grid tosupport die 30. Because die 30 is supported by a grid rather than by asolid planar surface, there is less surface area where the epoxy isforced between die 30 and lead frame 31. Less epoxy between thesupporting surface of the lead frame and the die reduces the effect theepoxy has on stability when the die is pressed against the epoxy. As aresult, both the chance of bubble formation and the amount of pressurerequired to seat the die uniformly on the lead frame are reduced,thereby allowing a more level positioning of the die relative to thelead frame and the package. It should be noted that the size and numberof cavities should be such that enough surface area exists on the leadframe to adequately secure the die, and the amount of epoxy used shouldbe such that excess epoxy is not squeezed along the sides of the die andonto the face when the die is placed and secured on the lead frame.

30.FIGS. 4A and 4B show another embodiment of the present invention,where FIG. 4A is a top view of a die 30 set on a lead frame 41, wherelead frame 41 is located within a package 32 and contains a single ovalor circular shaped cavity 43. FIG. 4B is a side view of FIG. 4A alongsectional line A-A′. Although shown as a general oval or circular shapedcavity 43, other suitably shaped cavities, such as rectangular, can alsobe used with this embodiment. Cavity 43 formed within lead frame 41 hasone dimension slightly smaller than the longer dimension of the die 30to be placed on the lead frame 41 so that die 30 rests on some portionof lead frame 41. Cavity 43 is shown in FIG. 4B as fully extendingthrough lead frame 41. However, if desired, cavity 43 can extend onlypartially into lead frame 41, as might be the case if too large a cavitywould structurally weaken the lead frame.

31. Cavity 43 is cut or formed prior to depositing die-attach epoxy 34,such as at the time the lead frame is manufactured. After cavity 43 hasbeen formed, die-attach epoxy 34 is deposited into cavity 43 within leadframe 41. The amount of die-attach epoxy 34 deposited should be suchthat the epoxy protrudes above the plane of the lead frame. Die 30 isthen placed on the top of epoxy 34 and pressed onto lead frame 41 by anysuitable placement mechanism. Because the epoxy 34 extends above theplane of the lead frame, epoxy 34 contacts the back surface of the die,and excess epoxy 34 is forced into unfilled portions of cavity 43underneath lead frame 41. If epoxy 34 has spread and filled cavity 43,additional excess epoxy 34 can spread out through openings 44 betweenthe edge of die 30 and the edge of cavity 43.

32. Because die 30 can be pressed and set directly on lead frame 41,without epoxy between die 30 and the upper surface of lead frame 41, die30 remains parallel with lead frame 41, and thus package 32.Furthermore, because the size of cavity 43 is slightly smaller than die30 so that only a small portion of die 30 is in direct contact with leadframe 41, a large amount of epoxy 43 is available to contact a largearea of die 30, thereby ensuring adequate adhesion. Consequently, diescan be packaged into lead frames with minimal or negligible positioningerrors caused by the die-attach epoxy.

33. The above-described embodiments of the present invention are merelymeant to be illustrative and not limiting. It will thus be obvious tothose skilled in the art that various changes and modifications may bemade without departing from this invention in its broader aspects. Forexample, any suitable number and shape of cavities can be formed withina lead frame in accordance with either of the two embodiments discussedabove to realize the benefits of the present invention. Therefore, theappended claims encompass all such changes and modifications as fallwithin the true spirit and scope of this invention.

We claim:
 1. A structure for packaging a die, comprising: a lead framehaving an upper surface and at least one cavity formed through saidupper surface, said at least one cavity underlying a portion of said diefor receiving excess die-attach epoxy when said die is adhered to saidlead frame.
 2. The structure of claim 1 , wherein said at least onecavity extends completely through said lead frame.
 3. The structure ofclaim 1 , wherein said at least one cavity partially extends into saidlead frame.
 4. The structure of claim 1 , wherein said lead framecomprises at least two cavities to form an array of said cavities. 5.The structure of claim 4 , wherein a die-attach epoxy is only depositedover said array.
 6. The structure of claim 4 , wherein at least onedimension of said array is slightly larger than at least one dimensionof said die.
 7. The structure of claim 4 , wherein said cavities arerectangular shaped.
 8. The structure of claim 1 , wherein said leadframe comprises a single cavity.
 9. The structure of claim 8 , whereinsaid cavity is circular shaped.
 10. The structure of claim 8 , whereinsaid cavity is oval shaped.
 11. The structure of claim 8 , wherein atleast one dimension of said cavity is slightly smaller than at least onedimension of said die.
 12. A method for packaging a die, comprising:forming at least one cavity formed through the upper surface of a leadframe; depositing epoxy over said at least one cavity; and placing saiddie over said epoxy and the upper surface of said lead frame, whereinexcess amounts of said epoxy flow into said at least one cavity.
 13. Themethod of claim 12 , wherein said epoxy is deposited so that a portionof said epoxy is above the upper surface of said lead frame.
 14. Themethod of claim 12 , wherein said at least one cavity extends completelythrough said lead frame.
 15. The method of claim 12 , wherein said atleast one cavity partially extends into said lead frame.
 16. The methodof claim 12 , wherein said forming forms at least two cavities to createan array of said cavities.
 17. The method of claim 16 , wherein saiddie-attach epoxy is only deposited over said array.
 18. The method ofclaim 16 , wherein at least one dimension of said array is slightlylarger than at least one dimension of said die.
 19. The method of claim16 , wherein said cavities are rectangular shaped.
 20. The method ofclaim 12 , wherein said forming forms a single cavity.
 21. The method ofclaim 20 , wherein said cavity is circular shaped.
 22. The method ofclaim 20 , wherein said cavity is oval shaped.
 23. The method of claim20 , wherein at least one dimension of said cavity is slightly smallerthan at least one dimension of said die.